1. Technical Field
The present invention relates to a three port optical filter device and, more particularly, to an optical filter utilizing a bulk filter element and a lensing arrangement to provide for adjustments in the filter characteristics.
2. Description of the Prior Art
It is well-known that thin-film bulk optic elements may be used as a filter device in optical communication systems. The orientation of the bulk optic element with respect to the light ray passing therethrough defines the wavelengths that are either reflected or refracted by the bulk optic element. Depending upon the materials used to form the film layers and the thicknesses of the various layers, a variety of different filter elements can be formed (i.e., bandpass, short wavelength pass, or long wavelength pass). A conventional two-port filter device may be formed that will pass any particular wavelength. A multiple wavelength system may be formed by combining, in parallel, a number of separate single wavelength filter devices. A three-port filter device may be formed by using a third waveguide to receive any signal reflected by the filter element. A problem with conventional three-port filter device designs is that any tilt of the filter element (often necessary to provide a "fine" adjustment of the filter's "pass" wavelength) results in changing the direction of the reflected signal such that the reflected signal may not be readily captured by the third waveguide.